Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

The primary stratigraphic fabric of a chaotic rock unit in the Zermatt Saas ophiolite of the Western Alps was reworked by a polyphase Alpine tectonic deformation. Multiscalar structural criteria demonstrate that this unit was deformed by two ductile subduction-related phases followed by brittle-ductile then brittle deformation. Deformation partitioning operated at various scales, leaving relatively unstrained rock domains preserving internal texture, organization, and composition. During subduction, ductile deformation involved stretching, boudinage, and simultaneous folding of the primary stratigraphic succession. This deformation is particularly well-documented in alternating layers showing contrasting deformation style, such as carbonate-rich rocks and turbiditic serpentinite metasandstones. During collision and exhumation, deformation enhanced the boudinaged horizons and blocks, giving rise to spherical to lozenge-shaped blocks embedded in a carbonate-rich matrix. Structural criteria allow the recognition of two main domains within the chaotic rock unit, one attributable to original broken formations reflecting turbiditic sedimentation, the other ascribable to an original sedimentary mélange. The envisaged geodynamic setting for the formation of the protoliths is the Jurassic Ligurian-Piedmont ocean basin floored by mostly serpentinized peridotites, intensely tectonized by extensional faults that triggered mass transport processes and turbiditic sedimentation.

Deformation partitioning in mountain belts: insights from analogue modelling experiments and the Taiwan collisional orogen

Many orogens on the planet result from plate convergence involving subduction of a continental margin. The lithosphere is strongly deformed during mountain building involving subduction of a plate composed generally of accreted continental margin units and some fragments of downgoing oceanic crust and mantle. A complex deformation involving strong partitioning of deformation modes and kinematics produces crustal shortening, accompanied by crustal thickening. Partitioning depends on three main factors: (1) rheologic layering of the lithosphere; (2) interaction between tectonics and surface processes; (3) subduction kinematics and 3D geometry of continental margins (oblique convergence, shape of indenters). Here we present an original view and discussion on the impact of deformation partitioning on the structure and evolution of orogens by examining the Taiwan mountain belt as a case study. Major unsolved questions are addressed through geological observations from the Taiwan orogen and insights from analogue models integrating surface processes. Some of these questions include: What is the role played by décollements or weak zones in crustal deformation and what is the impact of structural heterogeneities inherited from the early extensional history of a rifted passive continental margin? What is the relationship between deep underplating, induced uplift and flow of crustal material during erosion (finite strain evolution during wedge growth)? Are syn-convergent normal faults an effect of deformation partitioning and erosion? What is the role of strain partitioning on the location of major seismogenic faults in active mountain belts? What can be learned about the long-term and the present-day evolution of Taiwan?